Have you been using Time Machine to backup your Mac? Then restore a copy of you Photos Library from the Time Machine backup. when you restore the Photos Library, use the "Restore to ..." option to restore it to a different folder, so you do not overwrite the current Photos Library.
Restore a library from Time Machine in Photos on Mac - Apple Support
Restoring from a Time Machine backup can recover all photos that have been downloaded to the Mac in full resolution. If you have been using iCloud Photos with "Optimise Ma Storage" enabled, the optimised photos will be missing.